At least resource #3 got it right, but that definition comes from Indiana University while the rest of those resources are from your basic foo-foo fitness websites and personal trainers.
The constant in circuit training is this: the client should progress from one exercise to the next with minimal rest for the duration of the training session.

These are the variables in circuit training:

exercises

sets

reps

intensity

equipment

rest periods

duration

Changing any of those variables effectively changes the stimulus and benefits of the workout.

What we Already Know About the Physiology of Exercise

We have the following scientific facts about exercise physiology that we should look at when analyzing circuit training:

Anaerobic adaptions are facilitated by an overload stimulus.

Aerobic adaptions are facilitated by repetitive steady state aerobic exercise.

Aerobic exercise and anaerobic exercise cause different adaptations.

Cardiovascular benefits can be had by both anaerobic and aerobic exercise.

Are we looking to force a specific adaptation or just to exercise at a steady state?

Which type of muscle fibers are we looking to recruit?

Managing Energy Systems

If possible, we need to determine which energy system to target in order to force adaptations that will facilitate the client’s goals. The exercise program will target certain muscle fibers by stressing a particular energy system.

Intensity is the main factor that we will use to determine whether to use the aerobic or anaerobic energy system.

To review:

The anaerobic energy system will use a limited supply of Adenosine Tri-phosphate (ATP), phospho-creatine, carbohydrates, and protein when necessary to derive energy. The anaerobic energy system is used for high intensity, short duration activity where we can expect a maximal effort or shorts bursts of strength. Here, we focus on type IIb, fast twitch, anaerobic, glycolytic, white muscle fibers that contain only sparse amounts of mitochondria and myoglobin.

The aerobic energy system uses carbohydrates and fats for energy, and is mainly used for steady state activity, which includes simply being at rest. We must be able to maintain a steady oxygen concentration in the working muscles for this. Here we focus on type I short twitch fibers. Here, we focus on type I, slow twitch, aerobic, slow oxidative, red muscle fibers, which are full of capillaries and rich in mitochondria and myoglobin.

In this example we are trying to promote strength and muscle gains by including resistance training machines in the circuit.

Suppose we are performing a seated shoulder press (a common exercise in circuit training). We can choose to perform the exercise for a constant X number of reps regardless of intensity, or we can choose to rep until muscular failure before moving on to the next exercise.

In the case of working towards a constant X number of reps, we see that the intensity is not high enough to recruit enough fast twitch muscle fibers to force an anaerobic adaptation, nor are we focusing on the steady state aerobic exercise that would cause an aerobic adaptation. Instead, all we can hope to achieve is to burn some calories.

In the case of performing an exercise to muscular failure, we do engage the anaerobic energy system enough to encourage an adaptation, but in completing a full circuit we will surely find that our body can’t replenish ATP fast enough to be able to reach optimal muscular failure on each consecutive exercise (full replenishment takes a full 3 minutes on average). In this case we will see a decreasing return on energy invested. There is also no aerobic benefit to this protocol.

Here is another real world example:

In this example we include cardiovascular exercise in order to promote aerobic adaptation, but not necessarily fat loss.

Suppose we are using the treadmill for walking, jogging, and running. We can choose to walk or jog at a steady state for a constant X number of minutes, or we can choose to increase the intensity for 2 minutes for every 4 minutes of exercise in an attempt to imitate the HIIT training protocol.

In the case of working in a steady state, you will be able to encourage aerobic adaptation, but this is not circuit training, it is just plain cardio. True circuit training would have the client moving to various cardiovascular machines such as the stair master, elliptical, treadmill, rowing machine, etc… This would most likely provide the aerobic adaptations that the client is looking for, but would in NO way be conducive to muscle growth, which is a vital component of increasing metabolism – a short term and long term goal of any fat loss program. The anaerobic energy system does not apply in this case.

In the case of training in high intensity intervals, this is an excellent way to build muscle while burning calories, increasing aerobic capacity, and increasing metabolism. Once again though, this is not circuit training – this is HIIT training and involves the anaerobic energy system. This protocol could be implemented by prescribing a series of machines used at full intensity, alternating in combination with a series of plyometric or cardiovascular exercises (such as burpees or squat jumps), which would be used as the rest interval, but again this is NOT conducive to the steady state needed to facilitate the client’s goals of aerobic adaptation.

Learning From The Examples

As you can see with either of the above examples, we can focus on building muscle, strength gains, burning fat, or burning calories with circuit training, as long as we optimize all of the circuit training variables for each specific goal. But if you noticed, there is always a better solution to forcing the required adaptations, be it anaerobic weight lifting, HIIT, or long distance steady state endurance cardio.

For fat burning, we need to get to that steady state in order to specifically use fats as a fuel source. An individual can get to that steady state in roughly 3-4 minutes, but for aerobic adaptations to occur the steady state should be maintained for at least 20 minutes. Moving from station to station cause a variation of intensity and duration, and will often result in a sub-optimal use of the anaerobic energy system, causing little or no adaptation for either energy system. Fat burning in this case becomes highly unlikely.

So, What Is Circuit Training Really Good For?

We can draw several conclusions from the analysis of groups of individuals who have participated in studies involving prescribed circuit training for a prolonged period of time.

This is what we know about circuit training:

Circuit training is not optimal for increasing cardiovascular fitness when compared to High Intensity Interval Training (HIIT)

Circuit training is not optimal for increasing cardiovascular fitness when compared to cardiovascular training when the heart rate is at the target heart rate (THR) calculate your target heart rate

Circuit training is not optimal for forcing anaerobic adaptation when compared to a strength training program

Circuit training is not optimal for forcing aerobic adaptation when compared to an aerobic training program

Circuit training is not optimal for increasing muscle size when compared to a hypertrophy-based training program

Circuit training is not optimal for increasing strength when compared to HIIT or a regular resistance training program

Circuit training is useful for burning additional calories and stimulating blood flow to the limbs

Circuit training is useful for squeezing lots of exercise into a short time frame

Who Can Benefit From Circuit Training

In light of everything you just read, can anyone actually benefit from circuit training? The answer is yes, but obviously the benefits are not optimal for experienced athletes. Subgroups of trainees such as the elderly, young, rehabbing, novice, or obese athletes might do well circuit training optimized in the following ways:

Goal #1: maximal strength.

Clients looking to maximize anaerobic adaptations will have to use a circuit designed for high intensity, reaching a high level of fatigue on each exercise. Since there is a 3 minute threshold for complete ATP recovery, then a minimum of 2 minutes rest between sets is mandatory, or the required intensity and fatigue will not be generated for subsequent sets.

Goal #2: increase stamina.

As is the case with athlete who play sports such as football, MMA, or hockey, which require maximal force output in an exhaustive state, minimal rest periods would likely be beneficial. By maximizing intensity while also minimizing rest periods, circuit training with resistance machines is likely to increase stamina – repeated maximal force output without recovery.

Goal #3: burning calories and fat.

Since we are not able to burn fat unless we are working aerobically, most circuit training programs will not suffice unless each exercise is performed for a duration of at least 4 minutes. In this case we can only hope to burn calories, and possibly a little fat, by moving rapidly to each exercise station and maintaining a steady rhythm. Ultimately the client will be exercising not at, but close to, the anaerobic threshold.

Goal #4: fit as much exercise as possible in a limited time frame.

Circuit training does allow you to complete a high volume workout in a short period of time. A client with a restricted time frame, who just wants to exercise for the sake of exercise, without any specific goals, could benefit from circuit training. Little anaerobic or aerobic adaptations will occur, but the client will be in better condition and better health for having exercised at all.

Please note that by asking the client questions and ultimately arriving at a particular goal, you as a personal trainer should be able to devise an optimal workout routine that will force energy system adaptations, which will hasten the client to his or her goal faster than generic circuit training.

Specific aerobic or anaerobic workout routines will work much better than most forms of optimized circuit training. Remember this when designing a workout routine for yourself or your clients. Think about these things before settling your client into a generic circuit training routine.

4 Responses to “What is Circuit Training?”

Excellent post. Very detailed. However, one point that I thought should have been mentioned, and I did not see it mentioned, was muscle confusion. Muscle confusion will stimulate muscle growth faster than regular reps, and should be what the focus of circuit training is. Intense muscle confusion is the essence of the “300” body.

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